Abstract

The first half of tetrapod evolution witnessed substantial diversification of the clade and several major turnoversand mass extinctions. In the time since their origin, more than 380 Myr ago, to the beginning of theMiddle Jurassic 175 Myr ago, tetrapods apparently diversified fitfully, reaching their highest level in theMiddle Permian, and showing major diversity declines in the late Moscovian, Early Permian, Wordian,lower Wuchiapingian, end-Permian, lower Anisian, lower Ladinian, Late Triassic (lower Norian to upperRhaetian), end-Triassic, and Early Jurassic (upper Sinemurian, lower Pliensbachian). Of these diversitydrops, only the end-Permian and end-Triassic correspond to recognised mass extinctions, and the lateMoscovian and early Norian drops to other previously identified environmental crises. The remaindercould be real extinction or turnover events, or partially artefacts of biased sampling. There are strong correlationsbetween formation counts and tetrapod palaeodiversity, suggesting a sampling component in the rawdata, but the covariation is not uniform through the whole time span, being poor from Devonian to MiddlePermian, and better from Late Permian to Early Jurassic. There is limited evidence for covariation betweenthe tetrapod palaeodiversity time series and other putative sampling metrics, such as specimen completeness,numbers of publications, map areas, gap-bounded sedimentary units, rock volumes, formations, andfossil collections. Modelling by multiple correlations shows that formation count is generally the best explanatorymodel, either on its own, or combined with other ‘sampling’ time series. However, it is not clear thatformation count is independent of the palaeodiversity time series, because rises and falls in both signalscould reflect variations in original diversity or in preservation or in sampling.